Seismometer

ABSTRACT

A temperature compensating seismometer in which a spring supports a mass movable in a case wherein the effect of movement of the mass due to temperature changes on the spring is avoided by providing a fluid bypass between the fluid on opposite sides of the mass in the case, the bypass being sized to allow passage of fluid during slow movement of the mass, but sized to retard movement of the fluid during fast movement of the mass. A seismometer which provides a mechanical magnification of the signals to be measured by making the cross-sectional area of the mass larger than the cross-sectional area of the differential pressure measuring transducer. An improved differential pressure transducer in a seismometer which provides improved sensitivity.

United States Patent White [451 May 9,1972

[54] SEISMOMETER [72] Inventor: Gale White, 61 15 Crab Orchard, Houston,

Tex. 77027 [22] Filed: Apr. 30, 1970 [21] Appl. No.: 33,472

[52] US. Cl. ..340/17, 73/71, 340/8 LF Primary Examiner-Malcolm F.Hubler Assistant E.\'aminerR. Kinberg AtrorneyJames F. Weiler, JeffersonD. Giller, William A. Stout, Paul L. Deverter, ll, Dudley R. Dobie, Jr.and Henry W. Hope ABSTRACT A temperature compensating seismometer inwhich a spring supports a mass movable in a case wherein the effect ofmovement of the mass due to temperature changes on the spring is avoidedby providing a fluid bypass between the fluid on opposite sides of themass in the case, the bypass being sized to allow passage of fluidduring slow movement of the mass, but sized to retard movement of thefluid during fast movement of the mass. A seismometer which provides amechanical magnification of the signals to be measured by making thecrosssectional area of the mass larger than the cross-sectional area ofthe differential pressure measuring transducer. An improved differentialpressure transducer in a seismometer which provides improvedsensitivity.

6 Claims, 11 Drawing Figures P ATE NTEDMM 91972 3.662.327

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SEISMOMETER BACKGROUND OF THE INVENTION As indicated in applicantsapplication Ser. No. 306,796, filed Sept. 5, 1963, now abandoned, theusual method of detecting and measuring earth motion or movement is bythe use of a mass or inertia member suspended or supported by a springin a case. Suitable measuring means, such as a mechanical to electricaltransducer, is used to sense or measure the position of the mass withrespect to the case, the latter which is in contact with the earth.Thus, when the earth moves or vibrates, the mass tends to remain fixedwhile the case moves with the earth. The measuring means, generallyelectrical, delivers a signal corresponding to the earth movement due tothe varying position or relative movement of the case with respect tothe mass.

One feature of the present invention is the provision of a seismometerwherein as the mass moves relative to a case filled with a fluid, anincreased pressure is created on one side of the mass while a decreasedpressure is created on the second side of the mass and a pressuretransducer is provided in communication with the fluid on both sides ofthe mass for measuring the pressure differential which is an indicationof the relative movement of the mass to the case.

However, one problem with the general method of detecting earth movementwith a mass and spring is the effect of temperature changes on thespring which is used to support the mass. Most spring material willweaken or lose strength as the temperature rises, which allows the massto move downwardly relative to the case thereby giving an erroneoussignal. In addition, careful design of the measuring equipment isrequired to keep the displacement of the mass due to normal ambienttemperature changes to a range within the limits of the measuringcomponent. This is particularly true in the case of low frequencyseismometers where a long spring is required.

Furthermore, this limitation on the design of the measuring equipment,such as a transducer, restricts the efficiency of the transducer sincethe displacement of the mass due to temperature changes is usually muchlarger than the amplitude of the signals to be detected. Thus themeasuring means, such as the mechanical-electrical transducer, must becapable of accepting large displacements, and yet be sensitive to smalldisplacements.

SUMMARY The present invention is directed to the provision of atemperature compensating seismometer which eliminates the effects of themovement of the mass due to temperature changes thereby avoiding errorsignals caused by temperature changes and also allowing the use of anefficient transducer that is designed to operate at the very smallamplitude signals encountered by seismometers.

A still further object of the present invention is the provision of aseismometer which provides a mechanical magnification of the signals tobe measured.

A still further object of the present invention is the provision of aseismometer in which the ratio of the cross-sectional area of the massis larger than the cross-sectional area of a differential pressuretransducer thereby providing a signal magnification.

Still a further object of the present invention is the provision of animproved differential pressure transducer which provides improvedsensitivity for the measurement of the direction of movement as well asthe displacement of the mass in relationship to the case in aseismometer.

A still further object of the present invention is the provision of animproved differential pressure transducer in which two sets of threecondensers which vary in response to the differential pressure arearranged in T-circuits and are compared to provided a signal whichindicates the direction and magnitude of the differential pressure.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view, incross section, illustrating a preferred embodiment of the presentinvention,

FIG. 1A is a schematic, elevational view of reduced size, in crosssection, illustrating a modification of the present invention,

FIG. 2 is an enlarged fragmentary elevational view illustrating thedetails of construction of one type of suitable differential pressuretransducer that may be used with the present seismometer,

FIG. 3 is an enlarged elevational view illustrating the details ofconstruction of one of the movable condenser plates used in thetransducer of FIG. 2,

FIG. 4 is an enlarged elevational view of one of the fixed condenserplates used in the transducer of FIG. 2,

FIG. 5 is an electrical schematic diagram of a suitable electricalcircuit for connection to the transducer of FIG. 3 for obtaining asuitable measurement of the differential pressure and thus the directionand movement of the differential pressure,

FIG. 6 is an enlarged fragmentary elevational view, in cross section,illustrating a modified embodiment of a transducer,

FIG. 7 is an elevational view, illustrating the details of constructionof one of the condenser plates used in the transducer of FIG. 6, and

FIG. 8 is an electrical schematic diagram illustrating an electricalcircuit which may be used in conjunction with the transducer of FIG. 6to obtain suitable indication measurements,

FIG. 9 is a schematic elevational view of a modified form of theseismometer of the present invention, and

FIG. 10 is a schematic elevational view of still another modified formof the seismometer of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings,and particularly to FIG. 1, the seismometer of the present invention isgenerally designated by the FIG. 10. Generally, the seismometer 10includes a housing or case 12, an inertia member or mass 14 movable inthe case 12 and suspended therein by a spring 16, and a pressuretransducer 18 which measures the difference in pressure between thefluid above and below the mass '14. Generally, the case 12 is filledwith a fluid 20 such as oil or silicon fluid although any suitableliquid, gas or even air may be utilized. In use, the case 12 is incontact with the earth and thus when the earth moves or vibrates thecase 12 also moves or vibrates. The body of the mass 14 is slightlysmaller than the inside of the case 12 and when the apparatus 10 isproperly leveled the mass 14 will hang free from the spring 16 inside ofthe case 12. While the cross-sectional shape of the mass 14 and case 12may be any desired shape, they are preferably round. However, the mass14 tends to remain fixed when the case moves and therefore the relativemovement between the mass 14 and the case 12 is an indication of theamount of and direction of the earth movement or vibration. When thecase moves downward relative to the mass 14. the pressure in the fluid20 above the mass 14 will increase but the pressure below the mass willdecrease since the mass 14 tends to remain still and acts as a piston inthe case 12. The differential pressure transducer 18 measures thepressure differential between the fluid above and below the mass 14 andproduces a signal caused by this pressure difference which is anindication of the movement and direction of the mass 14 relative to thecase 12. Preferably, the pressure transducer 18 is connected to andmoves with the mass 14 thereby providing a quick response for anypressure changes in the fluid 20 and the case 12.

However, temperature changes will affect the strength of the spring 16and most spring material will weaken as the temperature rises therebycausing the mass 14 to move downwardly in the case 12. This causes anerroneous output signal. In addition, because the amplitude of signalsdetected by the temperature changes is usually much larger than theamplitude of the signals to be detected the efficiency of the transduceris reduced as it must be capable of accepting large displacements due tothe temperature changes, and yet be sensitive to the small displacementscaused by the earth movernents.

Therefore another feature of the present invention is the provision of abypass or slow leak-off path or restriction 22 in communication with thefluid above and below the mass 14, here shown in FIG. 1 as being betweenthe mass 14 and the case 12. The temperature changes, while of a largeamplitude, occur relatively slow as compared with the earth movement orvibrations which are to be measured. Therefore, the provision of thebypass or leak-off path 22 provides for fluid communication between thespace above and below the mass 14 so that pressure differentials causedby the temperature changes will, since they are relatively slowmovements, flow through the small clearance 22 and thus equalize thepressures above and below the mass 14. This eliminates any pressuredifferential in the case 12 and on the pressure transducer 18 due totemperature changes. However, the fluid bypass 22 is sized so as to retard the quick passage of fluid between the space above and below themass 14 whereby the signals to be measured of the earth movement orvibration, even though of a low amplitude, are at such a rate that apressure differential will be created above and below the mass 14 whichwill be detected by the pressure transducer 18 before the pressuredifference can be bled off through the bypass path 22.

Referring still to FIG. I it is noted that the motion magnification ofthe transducer is approximately the ratio of the square of the diameterof the mass 14 to the diameter of the pressure transducer 18. Therefore,it is desirable that the diameter or cross-sectional area of the mass 14be large as compared to the cross-sectional area or diameter of thediaphragm in order to magnify the pressure differential and thus thedetection signal.

In order to take care of the expansion and contraction of the fluid 20in the case 12 due to temperature changes, a small outlet 24 and areservoir is provided by a bellows 26.

While of course, any suitable differential pressure transducer may beutilized to measure the difference in pressure in the fluid above andbelow the mass 14, one preferred embodiment of such a transducer isshown in FIGS. 2-5. A transducer body 30, preferably cylindrical, isprovided connected to the mass 14 and held in place by lock-ring 32.Upper and lower diaphragms 34 and 36 are located at opposite ends of thetransducer and are exposed to the fluid pressure above and below themass 14, respectively. A connecting rod 38 is connected between thediaphragms 34 and 36 and thus moves in response, similarly to thediaphragms, to pressure differentials above and below the mass 14. Apair of movable condenser plates 40 and 42 are connected to aninsulating support 44 which is in turn connected and moves with theconnecting rod 38. On opposite sides and facing the movable condenserplates 40 and 42 are two stationary condenser plates 46 and 48 which aresuitably held in place by supports 50 and 52.

FIG. 3 shows a suitable shape for plate 46. Plate 48 is similar, andFIG. 4 shows a suitable shape for plate 40. Plate 43 will be similar.

Thus, the electrical capacity between the plates 40 and 46 and betweenthe plates 42 and 48 has a direct relationship to the movement of thediaphragms 34 and 36. And, of course, the movement of the diaphragmswill be related to the differential upper and lower pressures on eitherside of the mass 14. If the pressures are equal, the diaphragms will bein the center position and will in turn cause the capacity between theplates 40 and 46 and between the plates 42 and 48 to be equal. If thepressure above the mass 14 is larger than the pressure below the mass14, the movable plates 40 and 42 will move downwardly and this willcause the spacing between the plates 42 and 48 to decrease and sincecapacity is inversely proportional to the spacing between the plates,the capacity between plates 42 and 48 will increase. At the same time,an inverse action takes place between the movable plate 40 and the upperfixed plate 46, which results in a decrease of electrical capacitybetween these plates.

These changes in capacity may suitably be converted to an output signalby any desired electrical circuit. One such circuit is seen in FIG. 5 inwhich an oscillator 54 supplies a voltage, preferably at a highfrequency, for example I mc., across the condenser formed by the plates40 and 46 and the condenser formed by the plates 42 and 48 and thecircuit condensers 56 and 58. Condensers 56 and 48 are fixed and arechosen to equal the capacity between the variable condensers when thesystem is at rest and the movable plates are equally spaced between theupper and lower plates 46 and 48. Rectifiers 60 and 62 are provided toproduce opposite voltage outputs from the variable condensers formedbetween the plates 40 and 46 and 42 and 48, respectively. The outputsignals from the rectifiers 60 and 62 are connected to an adder 64 andwill result in a zero output when the movable plates are at their normalposition midway between the fixed plates 46 and 48. Now, as the movableplates move upwardly the capacitor formed by the plates 40 and 46becomes larger and the capacitor fonned by the plates 42 and 48 becomesmaller and due to the voltage dividing action of the condenser bridgecircuit the rectifiers will deliver signals to the adder 64. Since theoutput from rectifier 60 is larger than the output from rectifier 62 theadder 64 will yield a positive signal output. This will be an indicationof the magnitude and direction of the movement of the diaphragms 34 and36. Similarly, a negative output signal is developed when the movableplates 40 and 42 move downwardly.

Referring now to FIGS. 6, 7 and 8 a modified pressure transducer isshown wherein the capacitor plates are arranged to provide a greatersensitivity to displacement. For convenience of reference, themodification has the letter a applied to the parts corresponding tothose in FIGS. 2-5. The modification is to provide a network ofcapacitor elements all of which are controlled by the displacement ofthe diaphragms and which are arranged in a circuit to form a T-sectioncapacitor attenuator. Thus, two sets of three capacitors each areprovided between movable and fixed portions of the transducer. Thus,capacitors C1, C2 and C3 are formed with one set with plates 68 and 70forming capacitor C1, plates 72 and 70 forming capacitor C2, and plates74 and 76 forming capacitor C3, Similarly, a second set of capacitorsC4, C5, and C6 are formed with plates 78 and 80 forming C4, 82 and 80forming C5, and 84 and 86 forming C6. The two sets of capacitors arearranged in an electrical circuit as best seen in FIG. 8 in a mannersimilar to a push-pull circuit where one set will have an increase inattenuation while the other set has a decrease in attenuation for agiven displacement of the diaphragms and the connecting rod 38a. Forinstance, if the spacing between the capacitors Cl and C2 decreases whenthe diaphragms and the connecting rod 38a moves upwardly, the spacingfor capacitor C3 will increase and will cause a decrease in theattenuation of the oscillator voltage fed to rectifier 60a whichproduces an increase in the positive signal from the adder 64a. At thesame time, in the second set of capacitors, the spacing for C4 and C5will increase and the spacing for C6 will decrease, which will cause anincrease in the attenuation of the signal fed to the rectifier 62a,which produces a negative output. The result of this is a positiveoutput signal from the adder 64a which is an indication of the magnitudeand direction of the movement of the mass 14 relative to the case 12.The reverse takes place if the pressure transducer is moved in theopposite direction.

Of course, various modifications of the apparatus 10 may be provided.FIGS. 1A, 9 and 10 illustrate modifications out of the structureaccording to the invention, the letters a, b, and c, being applied tothe parts corresponding to those in FIG. 1 for convenience of reference.In FIG. 1A, the case 12a includes an opening 90 in the sidewall throughwhich a lever 92 is provided and which acts in conjunction with thespring 16a to support the mass 14 from a suitable support 94 and thusprovides a somewhat more stable supporting structure for suspending themass 14a in the case 12a. In this case the fluid on either side of themass 14a would be a gas such as air.

In FIG. 9 the pressure transducer 18b is connected to the interior ofthe case 12b with the inertia member or mass 14b suspended adjacent thetransducer 18b and spaced therein to provide the fluid bypass 22bbetween the mass 14b and the transducer 18b.

Referring now to FIG. 10 the piston 140 is suspended in the case 120 bythe spring 160 and acts as a piston therein and is provided with a fluidbypass 22c, all as shown in FIG. 1. However, an external fluidcommunication line 96 if provided in which is positioned the pressuretransducer 180 to measure the pressure differential on opposite sides ofthe mass 14c.

In use, and referring to FIG. 1, the mass 14 is suspended inside of thecase 12 and the apparatus 10 is then leveled so that the mass 14 willhang from the spring 16 freely inside the case 12. Preferably, a liquidsuch as oil or silicon fluid fills all of the void space inside of thecase 12 except for the interior of the pressure transducer 18. Thus,when the earth moves or vibrates the case 12 which rests on the earthmoves with the earth while the mass tends to remain fixed and thus thereis relative movement the mass 14 and the case 12. If the case 12 movesupwardly, the pressure will increase in the fluid above the mass and itwill decrease in fluid below the mass as the mass acts as a pistonbetween the two fluid compartments. The differential pressure transducer18 produces a signal caused by this pressure difference and transmits itover the signal leads 100 to suitable outside signal pickup equipment(not shown).

However, as previously mentioned the strength of the spring 16 willchange as the temperature changes thereby causing a vertical movement ofthe mass 14. In order to avoid erroneous readings caused by suchmovement and to allow the use of an efficient transducer which isdesigned to operate at very small amplitudes, usually desired forseismometers, a fluid bypass or leak-off path 22 is provided incommunication between the space above and below the mass 14 to allow thepressure to equalize. Bypass 22 is sized so that slow movements of themass caused by temperature changes allows the fluids to flow through thesmall path 22 and equalize the pressures on each side of the mass 14 andthereby avoid any pressure differential readings on the transducer 18caused by temperature changes. However, the small size of the passageway22 retards quick movement of the fluid about the mass 14, such as earthmovements or vibrations being measured, thereby causing a pressuredifferential on the transducer 18.

And as previously described in connection with the pressure transducershown either in FIGS. 2-5 or 6-8, movement of the diaphragms 34 and 36in one direction causes movement of the connecting rod 38 or 38a tocause a change in the electrical capacity of the condensers mountedbetween the movable and the fixed elements of the pressure transducerwhich is converted to a voltage output signal which indicates themagnitude and direction of the movement. Thus in FIGS. 2-5 when themovable diaphragms 34 and 36 and connecting rod 38 moves upwardly thecapacitor formed by the plates 40 and 46 becomes larger and thecapacitor formed by the plates 42 and 48 becomes smaller whereby thepositive signal delivered by the rectifier 60 to the adder 64 is greaterthan the signal delivered to the adder by the negative rectifier 62thereby indicating the magnitude and direction of the pressuredifferential and thus of the mass 14.

Similarly, the two sets of condensers shown in FIGS. 6-8 operate toprovide a negative and a positive signal which when balanced againsteach other shows the direction and magnitude of the movement. Thus, ifthe spacing between C] and C2 decreases when the diaphragms 34 and 36move upwardly the spacing for C3 will increase thereby causing anincrease in the positive signal from the rectifier 68 to the adder 64a.At the same time, in the second set of condensers, the spacing for C6will decrease which will cause a decrease in the signal fed from therectifier 62a to the adder 64a which results in a positive output signalfrom the adder 64a which may'thenbe transmitted through suitableelectrical leads (FIG. 1) to a suitable measuring apparatus (not shown).

Similarly, the seismometer embodiments shown in FIGS. 1A, 9, and 10 aresuitably actuated to measure a-pressure-differential reading as the mass14a, 14b and 14c move relative to the case 12a, 12b, and 12c,respectively. Similarly, fluid bypass or bleed-off passages 22a, 22b and22c may be provided to provide a temperature compensating seismometer aspreviously described. Any suitable pressure transducer 18a, 18b, and180, respectively may be used, and the embodiments shown in FIGS. 2-5and 6-8 produce a suitable output signal which provides a sensitivesignal indicating both the direction and magnitude of the mass change.

It is also to be noted that the differential pressure transducers shownin FIGS. 2-5 and 6-8 may be used to detect and measure differentialpressures in uses other than the seismometer of the present inventor andwill provide an efficient and sensitive measurement.

The present invention, therefore, is well adapted to carry out theobjects and attain the ends and advantages mentioned as well as othersinherent therein.

What is claimed is:

l. A temperature compensating seismometer comprising,

a case,

a mass movable in said case,

a spring supporting the mass in the case intermediate the ends of thecase,

fluid in said case on opposite sides of said mass,

a fluid bypass communicating between the fluid on opposite sides of themass and sized to allow the passage of fluid due to slow movement of themass but sized to retard passage of fluid due to fast movement of themass, and

a pressure transducer in communication with the fluid on opposite sidesof the mass for measuring the pressure differential on the oppositesides of the mass.

2. The invention of claim 1 wherein,

the fluid bypass is between the mass and the case.

3. The invention of claim 1 wherein the transducer is within andconnected to the mass.

4. The invention of claim 1 wherein the cross-sectional area of the massis larger than the cross-sectional area ofthepressure transducer.

5. A temperature compensating seismometer comprising,

a case,

a mass movable in said case,

a spring supporting the mass in the case intermediate the ends of thecase,

fluid in said case on opposite sides of said mass,

a fluid bypass communicating between the fluid on opposite sides of themass and sized to allow the passage of fluid due to slow movement of themass but sized to retard passage of fluid due to fast movement of themass,

a pressure transducer in communication with the fluid on opposite sidesof the mass for measuring the pressure differential on the oppositesides of the mass,

said pressure transducer including two diaphragms, the first incommunication with fluid on one side of the mass, and the second incommunication with'the fluid on the second side of the mass,

a pair of condenser plates connected to and movable with saiddiaphragms,

a fixed condenser plate on each side of said pair of plates forming apair of condensers whereby the electrical capacity of one of thecondensers increases while the capacity of the other condenser decreaseswhen the movable plates and diaphragms are moved, and

an electrical measuring circuit including,

an oscillator connected to the pair of condensers,

a rectifier connected to the output of each condenser,

and means comparing the outputs from said vrectifiers. 6. A temperaturecompensating seismometer comprising,

a case,

a mass movable in said case,

a spring supporting the mass in the case intermediate the ends of thecase,

fluid in said case on opposite sides of said mass,

a fluid bypass communicating between the fluid on opposite sides of themass and sized to allow the passage of fluid due to slow movement of themass but sized to retard passage of fluid due to fast movement of themass,

a pressure transducer in communication with the fluid on opposite sidesof the mass for measuring the pressure differential on the oppositesides of the mass,

said pressure transducer including two diaphragms, the first incommunication with fluid on one side of the mass, and the second incommunication with the fluid on the second side of the mass,

two sets of variable condensers connected to said diaphragms, the firstset having two condensers in which the capacity increases and onecondenser in which the capacity decreases when the diaphragms move inone direction, the second set having two condensers in which thecapacity decreases and one condenser in which the capacity increaseswhen the diaphragms move in said one direction, an electrical signalcircuit including,

an oscillator connected to each set of condensers, each set ofcondensers arranged in a separate T-circuit with the one condenser beingin the leg of the T, and means for comparing the outputs of each set ofcondensers.

1. A temperature compensating seismometer comprising, a case, a massmovable in said case, a spring supporting the mass in the caseintermediate the ends of the case, fluid in said case on opposite sidesof said mass, a fluid bypass communicating between the fluid on oppositesides of the mass and sized to allow the passage of fluid due to slowmovement of the mass but sizEd to retard passage of fluid due to fastmovement of the mass, and a pressure transducer in communication withthe fluid on opposite sides of the mass for measuring the pressuredifferential on the opposite sides of the mass.
 2. The invention ofclaim 1 wherein, the fluid bypass is between the mass and the case. 3.The invention of claim 1 wherein the transducer is within and connectedto the mass.
 4. The invention of claim 1 wherein the cross-sectionalarea of the mass is larger than the cross-sectional area of the pressuretransducer.
 5. A temperature compensating seismometer comprising, acase, a mass movable in said case, a spring supporting the mass in thecase intermediate the ends of the case, fluid in said case on oppositesides of said mass, a fluid bypass communicating between the fluid onopposite sides of the mass and sized to allow the passage of fluid dueto slow movement of the mass but sized to retard passage of fluid due tofast movement of the mass, a pressure transducer in communication withthe fluid on opposite sides of the mass for measuring the pressuredifferential on the opposite sides of the mass, said pressure transducerincluding two diaphragms, the first in communication with fluid on oneside of the mass, and the second in communication with the fluid on thesecond side of the mass, a pair of condenser plates connected to andmovable with said diaphragms, a fixed condenser plate on each side ofsaid pair of plates forming a pair of condensers whereby the electricalcapacity of one of the condensers increases while the capacity of theother condenser decreases when the movable plates and diaphragms aremoved, and an electrical measuring circuit including, an oscillatorconnected to the pair of condensers, a rectifier connected to the outputof each condenser, and means comparing the outputs from said rectifiers.6. A temperature compensating seismometer comprising, a case, a massmovable in said case, a spring supporting the mass in the caseintermediate the ends of the case, fluid in said case on opposite sidesof said mass, a fluid bypass communicating between the fluid on oppositesides of the mass and sized to allow the passage of fluid due to slowmovement of the mass but sized to retard passage of fluid due to fastmovement of the mass, a pressure transducer in communication with thefluid on opposite sides of the mass for measuring the pressuredifferential on the opposite sides of the mass, said pressure transducerincluding two diaphragms, the first in communication with fluid on oneside of the mass, and the second in communication with the fluid on thesecond side of the mass, two sets of variable condensers connected tosaid diaphragms, the first set having two condensers in which thecapacity increases and one condenser in which the capacity decreaseswhen the diaphragms move in one direction, the second set having twocondensers in which the capacity decreases and one condenser in whichthe capacity increases when the diaphragms move in said one direction,an electrical signal circuit including, an oscillator connected to eachset of condensers, each set of condensers arranged in a separateT-circuit with the one condenser being in the leg of the T, and meansfor comparing the outputs of each set of condensers.